186 EMBEYOLOGY OF THE LOWEE VEETEBEATES oh. 



maining for a time as a longitudinal groove opening ventrally 

 towards the yolk. As the lips of this groove gradually coalesce at 

 each end the communication between the gut cavity and the yolk 

 becomes gradually narrowed down to the tubular cavity of the yolk- 

 stalk situated at first behind the liver but later becoming' shifted 

 forwards by differential growth. Eventually this becomes obliterated 

 and the definitive alimentary canal becomes completely isolated from 

 what remains of the yolk. In many Teleostean fishes this isolation 

 takes place at a very early stage in development. 



The alimentary canal is, in correlation with its digestive function, 

 necessarily a highly glandular organ. Primitively the secretory 

 functions are carried out by unicellular glands, scattered about 



amongst the other epithelial cells 

 of the endoderm, but in the Verte- 

 brates, as in all the more complex 

 Metazoa, special concentrations of 

 gland cells and of secretory activity 

 take place in localized portions of 

 the enteric wall. Each of these 

 specially glandular patches under- 

 goes a great increase in its area, 

 which causes it to bulge outwards 

 as a simple or much subdivided 

 and complicated pocket, forming 

 a distinct glandular appendage of 

 the alimentary canal. 



Livee. — Of these glandular- 

 appendages, in the case of Verte- 

 brates, the most ancient appears to 

 be the liver, which is already present in Amphioxus. In this animal 

 the liver originates in ontogeny (Hammar, 1893) as a pocket-like 

 outgrowth of the alimentary canal wall on its ventral side and 

 slightly posterior to the hind end of the pharynx. Apart from 

 increase in size and relative narrowing of its base of attachment the 

 liver in Amphioxus undergoes no further complication but retains 

 its extraordinarily primitive pouch-like condition throughout life. 



In the holoblastic Craniates the liver arises similarly as a ventral 

 projection of the alimentary canal wall. This shows the customary 

 modifications in correlation with the presence of yolk, arising in some 

 cases in the more primitive fashion as a hollow pocket (Lampreys, 

 many Amphibians, Ceratodus), in others (many Amphibians, Lepido- 

 siren and Protopterus) as a solid knob of yolk-laden cells (Fig. 105, 

 li). This grows rapidly in size, as it uses up its food-yolk, and 

 becomes constricted off from the main mass of yolk by ingrowing 

 mesenchyme, until its attachment becomes narrowed down to a 

 slender stalk — the rudiment' of the bile-duct. 



The pouch-like rudiment of the liver undergoes an active process 

 of sprouting into numerous secondary pockets, each of which becomes 



Fig. 107. — Transverse section through hind 

 portion of intestine of a larva of Ich- 

 thyophis. (After Sarasins, 1889.) The 

 stage of development was that shown in 

 Fig. 104, F. The sheath of splanchnic 

 mesoderm is omitted. 



